Process for heating and sintering ferrous powder metal compacts

ABSTRACT

An induction field is utilized for rapidly raising the temperature of a ferrous powder metal compact to a sintering temperature where the compact is deoxidized and its particles coalesced into a homogenous mass. In order to overcome a characteristic incubation period which delays initial increase in temperature of such a compact in an induction field, the outer surfaces of the compact are preheated for a relatively short time by known heating means to raise its surface temperature to about 400* F prior to subjecting the compact to a magnetic induction field. After the preheating step, heating in the induction field is used to raise the temperature of the entire compact to a level above 1,800* F, and this temperature is maintained for a sufficient period of time to deoxidize and sinter the compact. After the heating and sintering treatments, the compact is cooled to a temperature of about 1,500*-1,900* F for a final forming operation which establishes a useful, full, dense, finished shape.

Unite States Patent 1 1111 3,779,747

Qorita Dec. 18, 1973 PROCESS FOR HEATING AND SINTERING Primary Examiner-Carl D. Quarforth FERROUS POWDER METAL COMPACTS Assistant ExaminerR. E. Schafer [75] Inventor: Robert L. Conta, Rochester, NY. Am)me'v MOn0n Polster et [73] Assignee: 'lghs Gleason Works, Rochester. [57] ABSTRACT An induction field is utilized for rapidly raising the [22] plied Sept 1972 temperature of a ferrous powder metal compact to a [21] Appl. No.: 285,965 sintering temperature where the compact is deoxidized and its particles coalesced into a homogenous mass. In order to overcome a characteristic incubation .5 Cl 09 29/4205 75/2l4 period which delays initial increase in temperature of 75/226 9/1041 such a compact in an induction field, the outer sur- [Sil hit. Cl 322i 3/24 faces of the compact are preheated for a relatively [58] Field Of Search 75/200, 226, 214; Short time y known heating means to raise its Surface 219/1041; 29/420'5 temperature to about 400 F prior to subjecting the compact to a magnetic induction field. After the pre- [56] References Cted heating step, heating in the induction field is used to UNITED STATES PATENTS raise the temperature of the entire compact to a level 3,708,645 1 1973 Osborn 219/1041 above 1,8 0 F and this temperature is maintained for 3,331,686 7/1967 Bonis ct al. 75/226 a sufficient period of time to deoxidize and sinter the OTHER PUBLICATIONS compact. After the heating and sintering treatments, k C H F G P d M P the compact is cooled to a temperature of about foiiiis in Machiriiiz anci i soddziion iiiigi rieeri rig ll 1500049006 F for a final forming Operation which pp 6684,70, 1970- T. M17 establishes a useful, full, dense, finished shape.

21 Claims, 5 Drawing Figures I II III IV V i l l G PREHEATING INDUCTION FURTHER ADJUSTING FINAL giiiiiiis To A "age i2... TA SURFACE TEMP. BOV POWDER ME L OF AT LEAST (30-120 SEC.) |800F TO (30-60 SEC.) COMPACT 300F DEOXIDIZE (30- 60 SEC.) AND SINTER (2-5 MIN.)

PROCESS FOR HEATING AND SINTERING lFERlROUS POWDER METAL COMPACTS BACKGROUND OF INVENTION The present invention particularily relates to processes for heat treating and sintering powder metal compacts to prepare the compacts for final forming operations which increase the density and establish the final shapes of the compacts as finished parts, but the discoveries of this invention may be applied to processes which sinter powder metal parts as a final step in a finishing technique. The invention is specifically concerned with improvements in heating and sintering of compacts formed from ferrous metal powders used in the production of high strength finished parts such as bevel and spur gears, and the like.

Various processes are known for initially compacting a cold metal powder into a coherent form which can be more easily handled during required treatments of heating and sintering prior to final forming or forging operations. Typically, a precisely measured quantity of the metal powder is compacted by isostatic pressure, or by mechanical die means, into an initial shape having characteristics of increased density and coherency. The compact form is then subjected to heating and forming treatments required for driving off lubricants or binders, coalescing particles of the compact, and increasing density and shape of the compact into a finished part.

it is also known to raise the temperature of powder metal compacts to a level at which individual particles of the compact coalesce into a homogenous mass without actual melting of the particles (also described as diffusion bonding). This treatment is commonly carried out at about 2,050 F with known sintering equipment. However, it has been recently discovered that somewhat higher temperatures can be used for effecting coalescence of particles of a powder compact and for simultaneously deoxidizing the compact. In the context of this specification and its claims, references to sintering or sintering temperatures are intended to define deoxidation of a compact as well as the formation of a homogenous mass from a metal powder.

Various processes and means have been attempted for carrying out efficient sintering of powder metal compacts so that the compacts will be in a condition to be formed in a die to increase their densities to nearly 100 percentof theoretical density and to establish a final shape of a finished part. Known processes for heating and sintering compacts prior to final forming have been somewhat inefficient or uneconomical because of excessive heating times or energies required to carry out the necessary conditioning of a compact for final forming. For example, it is known to sinter a compact with a radiant heating furnace to a level of about 2,050 E for a relatively long period of time of about twenty minutes in order to deoxidize and sinter the powder metal of the compact. It is also known to utilize an induction field of heat bar stock, and the specification of French Pat. No. 2,050,096 suggests the use of induction heating for a certain combination of ferrous metal chips and powder. However, it is believed that there has been no commercially successful and economical use of induction heating for heating and sintering a powder metal compact having a ferrous metal composition to produce high strength metal parts, such as gears. Induction heating is an interesting possibility for raising the temperature of a ferrous metal powder compact because it permits a controlled addition of substantial energy into the body of the compact to thereby raise its temperature in a relatively short time. However, it has been discovered that when a readily available low frequency (3KH for example) induction field is utilized as a means for raising the temperature of a compact to a sintering temperature level, there is a characteristic incubation" period at the onset of each heating cycle in which there appears to be a de' layed temperature response in the induction field. This delay is significant, sometimes amounting to several minutes depending upon the size and density of the compact, which adds considerably to the cost of energy required for an induction heating treatment. It is not known precisely why a powder compact exhibits an incubation period at the beginning of an induction heating cycle, but it is apparent that the compact is very inefficient in absorbing power and that initial heating is slow in an induction field. Whatever the reason for the incubation period, it has been noted that compacts formed by isostatic compaction methods exhibit a more pronounced incubation period than those produced in die presses by mechanical means. Accordingly, there is a need for an improved process for taking advantage of presently available induction heating equipment in a sintering operation.

BRIEF DESCRIPTION AND SUMMARY OF INVENTION The present invention provides for improvements in heat treating processes for ferrous metal powder compacts so that such compacts can be economically and efficiently heat treated in a continuous process which carries each treated compact to a final forming operation where its density is increased to approximately percent of theoretical value and its shape is established for a finished part. In addition, the present invention provides for an overall decrease in time and energy required for heat treating such compacts through a use of induction heating in an efficient and economical manner for at least a part of the heat treatment of each compact.

The improved process of the present invention is based upon a discovery that the incubation period (or the time it takes for the compact to effectively couple with a magnetic field of an induction heating system) of a given compact in a relatively low frequency induction field can be substantially reduced or eliminated by preheating only the outside surfaces of the compact to a temperature level which produces an irreversible change in characteristics of the compact with respect to its ability to respond to the induction heating field. It is not known just what mechanical or chemical changes take place when a compact is preheated prior to introduction into an induction heating field, but it can be observed that the preheated compact responds much more readily to induction heating, thereby significantly reducing the total power required in an induction heating cycle to raise the temperature of the compact to a sintering level. Preheating can be carried out with known radiant heating equipment in a very short time which raises the temperature of only the skin surfaces of the compact prior to induction heating. The surface temperature is raised to a level above 300 F, and there appears to be a preferable threshold level of about 400 F at which incubation time is nearly eliminated, but somewhat lower temperatures may be utilized if the preheating treatment is carried out for a longer period of time. In a commercial application of the discoveries of the present invention, it is comtemplated that preheating of a 295 gram compact will require about one minute or less to raise its surface temperature to the preferred 400 F level. After this preheating treatment, the compact can be immediately introduced into an induction heating field, or it can be cooled and stored for a subsequent use in an induction heating field since the effect of preheating appears to be an irreversible one with respect to reduction or elimination of the characteristic incubation period for a compact in an induction field.

Thus, the basic process of the present invention is one of increasing the rate at which ferrous metal powder compacts can be efficiently heated in an induction heating field by preheating the powder metal compact sufficiently to raise the temperature level of the material in its outer surfaces to at least 300 F prior to subjecting the compact to an induction heating field which then can rapidly raise the temperature level of the material of the entire compact to a level at which sintering takes place.

An overall process for forming ferrous metal powder compacts into high strength finished parts includes the steps of (a) preheating each compact sufficiently to raise the temperature level of material in its outer surfaces to at least 300 F (and preferably to about 400 F), (b) subjecting the compact to an induction heating field which rapidly raises the temperature of the material of the compact to a sintering level (between I,800F and about 2,500 F for a ferrous metal), (c) maintaining the temperature of the compact at the sintering level for a sufficient length of time to deoxidize the compact and to cause powder particles therein to coalesce, and (d) cooling the compact from said sintering level to a temperature at which the compact can be immediately formed into a finished part 1,500 l,900 F). Step (d) can be omitted for processes in which sintering produces thefinal form of the article.

Although this specification will emphasize the importance of preheating of a powder metal compact in a commercial process which utilizes induction heating as a means to reduce heating and sintering times, it should be recognized that other variables affect the incubation period exhibited by powder compacts in an induction field. For example, it has been observed that if compact density is maximized, incubation time is reduced. This would suggest a use of denser compacts in an overall process. In addition, it has been determined that increased induction power density and frequency re duces incubation time in an induction field. Thus, these additional variables should be considered in an overall process for heating and sintering powder metal compacts and may be combined with the advantages obtained by the type of preheating proposed by the present invention.

These and other features and advantages of the invention will become apparent in the detailed discussion which follows. In that discussion reference will be made to the accompanying drawings as briefly described be low.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a sehematic illustration ofa continuous process by which a compact of ferrous powder metal is prepared and treated in accordance with the present invention for use in a final forming operation;

FIG. 2 is a temperature-time graph which depicts the various stages of heating and sintering treatments suggested for the process of FIG. ll;

FIG. 3 is a graph illustrating reduction in incubation period for compacts subjected to preheating for different time durations;

FIG. 4 is a graph illustrating the effects of various durations and temperatures of preheating on incubation time;

FIG. 5 is a graph showing a temperature-time profile which indicates a range of preheat times and tempera tures that will nearly completely eliminate incubation time for a 295 gram compact.

DETAILED DESCRIPTION OF INVENTION FIG. 1 depicts the invention with reference to an overall continuous process for producing finished metal parts from a ferrous metal powder. FIG. 2 graphically illustrates the same continuous process shown in FIG. ll. This process is especially useful in producing high strength gear forms which exhibit equal or superior characteristics to present day gears manufactured by a method of cutting and removing material from a blank. The gear forms which can be produced by the process of the present invention include various known bevel gears as well as spur gears and other types of gear designs and gear tooth forms.

Station I of FIG. 1 comprises a place at which a ferrous metal powder is compacted at ambient temperatures to increase its density and to create a coherent preform or compact which can be easily handled in subsequent steps of heating and forming. The compacting technique does not form a separate part of the present invention and may consist of any known isostatic or mechanical method for increasing metal powder density to about percent to 90 percent of its theoretical full density. Prior to compacting, the ferrous metal powder is selected or prepared to provide for a controlled admixture of carbon for reacting with combined oxygen and for forming an alloy with the ferrous metal of the powder. Preferably, the powder is prepared in accordance with techniques described in co-pending application Ser. No. 190,353, filed on Oct. 18, 197i in the name of Philip J. Guichelaar under the title Method For Producing High Strength Finished Forms From Ferrous Metal Powders, and commonly owned with the present application. This technique suggests, for example, the use of a commercially available, water atomized, annealed metal powder having a high ferrous content of about 98 percent by weight of the powder. About 0.20 to 0.60 percent by weight of graphite is admixed with the metal powder, and the mixture is subjected to isostatic compacting to produce a preform or compact having about to percent of the theoretical full density for the selected metal. It is not essential that a lubricant or binder be added to a powder mixture when isostatic techniques are used for compacting, and therefore, a subsequent step for removing such a lubricant or binder is not required, as is the case with certain other compacting methods.

After compacting, the preform is delivered to suitable apparatus for carrying out a series of heating treat ments in accordance with the present invention. These heating treatments may be included as a part of a continuous process which provides for continuous delivery of compacted preforms from a compacting machine to the heating apparatus which will carry out critical steps of heating as the preforms move through a series of heating zones within the apparatus. Stations ll, ill, IV and V represent the stations at which heating treatments will be applied to compacts moving through heating zones contained therein. Upon completion of the heating treatment of this invention, the hot compacts will 'be rapidly delivered to 'a final forming or forging press at station VI. Again, final forming can be included as part of an overall continuous process, and means would be provided for unloading hot compacts from the final heating station V for a rapid transfer into the forming press at station VI. A continuous process of the type depicted in FIG. 1 can be carried out in a time period of about 6 to 10 minutes (for relatively small gear parts) from the time of initial compacting until a finished form is produced at station VI.

The heating and sintering stations ll V provide for certain treatments which are necessary to carry out reactions and physical changes in a powder metal compact before it can be formed into a high strength finished product. The stations ll V are arranged to maintain an inert (or reducing) atmosphere around each compact as the compact advances from station-tostation, Induction heating is used at staion Ill to add substantial heat energy to a powder compact in a relatively short period of time so as to bring the material of the compact to a sintering temperature at or above 1,800 F. However, it has been observed that ferrous metal compacts exhibit a characteristic incubation period at the beginning of a heating treatment in a low frequency induction field, and this period is of sufficient duration to resuit in a costly expenditure of energy without attainment of a desired rapid change in the temperature of the compact. in accordance with the present invention, the incubation period is substantially reduced or eliminated by subjecting each compact to a preliminary step of preheating prior to the step of induction heating. Preheating is carried out at station ll with known radiation heating equipment, for example, which functions to raise the surface temperature of the compact to at least 300 F but less than the sintering temperature which will be attained in the induction heating zone. By preheating only the outside surfaces of the compact, it is possible to reduce the time required for preheating, and yet, a very desirable and apparently irreversible change is effected by such treatment whereby the compact will then respond to relatively low frequency (at least 1 KH induction heating with very little or no incubation period delay. It is also desirable to preheat only skin surfaces of the compact to avoid an unnecessary stressing that may resuit in cracking of the compact during subsequent heating and sintering operations. Although preheating can be carried out over a relatively broad range of temperatures for durations of time which generally decrease as the temperature is increased, it is preferred that the surface area of a compact be raised to a temperature of at least 400 F to obtain a relatively rapid preheating treatment at a reasonable cost.

As an example of a preheating treatment of ferrous powder compacts, compacts weighing approximately 295 grams were placed in a small resistence-type tube furnace operating with a wall temperature of approximately l,500 F. The clearance between each compact and the 1,500 F wall was approximately oneeighth inch, and separate batches of the compacts were pre heated for times varying between 0.5 and 1.5 minutes after which they were immediately moved into a 3 Kl-l induction field. FIG. 3 shows the reduction in incubation period for com acts preheated at 0.5, 1.0, and L5 minutes, in the type of radiation heating equipment just described, and as thereafter subjected to induction heating at power densities of 1.6, 3.2 and 4 8 KW/inF. Compacts which were not preheated are als o indicated on the graph of FIG. 3 to illustrate the substantial reduction in incubation time for various combinations of preheating and induction heating at different power densities. it can be seen that incubation time is reduced as preheating time is increased and as the power density of the induction field is increased.

After preheating, the compact can be moved directly to the induction heating station Ill where it is subjected to an induction field which has the effect of raising its temperature to a sintering temperature level. Although sintering can be carried out at about 1,800 F, it is preferred, in certain processes, that a higher temperature, on the order of 2,350 F be used to assure a full deoxidation reaction of the compact to remove at least percent of combined oxygen from the powder mixture. The desired deoxidation reaction, together with coalescence of powder particles, can be achieved by maintaining the sintering temperature for approximately 2 to 5 minutes after the temperature is reached. This can be done by continuing the induction heating of the compact or it can be done by moving the compact to a separate heating station [V which utilizes radiant or other known heating means to maintain the sintering temperature. It is advantageous to use radiant heating for maintaining the sintering temperature since substantial cooling of an induction coil may be required if induction heating is used for the same purpose, and this could result in unnecessary and unwanted thermal losses from the system. Thus, it is preferred in many application to utilize induction heating to rapidly raise the temperature of a compact up to the sintering level and to then use radiant or other heating for maintaining the temperature at such level for a sufficient period of time to carry out all required sintering reactions. Sintering is carried out at a temperature less than the actual melting point of the powder compacts, and preferably below 2,500 F.

After sintering, the temperature of the compact must be adjusted prior to forming since the compact is not in an ideal condition for forming at the sintering temperature level.

Forming is preferably carried out at a temperature of l,500l,900 F, and this requires a cooling of the entire compact. Since outside surfaces of a large compact tend to cool more rapidly than its inner core areas, it is necessary to cool the outside surfaces to a temperature below that required for forming, followed by a reheating of the outside surfaces to the desired forming temperature. Thus, the cooling step of the process can be considered a temperature adjusting step since there may be a step of reheating involved in reducing the temperature of the sintered compact down to a forming temperature. Cooling and temperature adjusting take place at station V with known means for transferring heat from hot compacts as they pass through the sta tion. After cooling to about 1,500 to l,900 F, the compacts are immediately transferred to a forming press for final forming operations which increase the density of the compacts to about 100 percent of theoretical density for comparable bar stock.

FIGS. 3 through 5 portray certain relationships determined from the heat treating process of the present invention as applied to a ferrous metal powder isostatically compacted to a nominal density of 6.3 grams per cc for each compact tested. Each compact weighed 295 grams, and the powder used was of a water atomized type marketed by A. O. Smith-Inland'as 4,600 powder with 0.35 percent graphite added. The properties of this type of powder are represented in Table 1 below:

TABLE I Representative Powder Properties A. Chemistry:

C 0.01% Mn 0.16% P 0.009% S 0.0l6% Si 0.02% 0 O. l 0% Ni 1.86% M0 0.50% Fe balance B. Sieve Analysis (U. S. Standard):

80 +l00 2.1% l00 +l50 17.4% 150 +200 23.7% 200 +250 7.2% 250 325 20.8% 325 28.8%

The powder compacts produced as above were subjected to a range of preheating temperatures ranging from 200 F to l,000 F (temperature of skin surface of compact at end of preheating treatment), and compacts within each temperature range were heated for time periods varying from one minute to twenty minutes. The effects of these various preheating treatments are depicted in FIG. 4 wherein line 10 represents a plotted line determined from preheating times of l, 5, l0, l5, and minutes to a compact surface tempera ture of 200 F. Similarly, lines I2, 14, 16, 18, 20 and 22 of FIG. 4 represent averages obtained for preheating of compacts to the higher temperatures indicated on each line for the periods of time which can be plotted from the graph. The relationship of preheating to incubation time can be seen in terms of a substantial reduction in incubation time as preheating temperatures are increased. However, the results of the tests depicted in FIG. 4 would suggest that a commercial process would prefer a minimum preheating temperature of a com pact to about 300 F, although temperatures in the range of 400 to 500 F offer even more noticeable reductions in incubation time. Incubation time was determined by rapidly moving a preheated compact to an induction field having a constant power density of 3.2 KW/in. and the end of the incubation period was defined to be when a 2 percent shift in power factor from the unincubated state had occurred between the power supply and the induction tank circuit.

FIG. 5 represents a temperature-time profile which shows a range of preheat times and temperatures which will produce a nearly complete elimination of incubation time for a 295 gram compact. The enclosed area of the graph of FIG. 5 indicates a range of time and preheat temperature combinations which produce approximately 400 F surface temperatures for the compact material. Incubation time within the enclosed area of the graph was nearly zero for induction heating treatment which included power densities at 1.6, 3.2, and 4.8 Kw/in In addition to the above determinations, compacts have been preheated and allowed to cool to room temperature to test for reduction in incubation time in an induction field. It has been observed that such preheating and cooling, followed by rapid reheating in an induction field exhibits the same reduction or elimination in incubation time as determined above for compacts which were moved immediately from preheating into the induction field. This indicates an apparently irreversible change in responsiveness of the compact to induction heating after being subjected to the type of controlled preheating required by this invention. Thus, it would be possible to pretreat powder metal compacts to prepare them for a later use in induction heating and sintering treatments of the type discussed above.

Although the invention has been described above with reference to a specific ferrous metal powder and specific treatment steps applied thereto, it can be appreciated that the principles of this invention can be utilized in improving characteristics of other ferrous metal compositions which are to be subjected to induction heating processes. Of course, composition, or other characteristics, of a particular powder metal selected for use will change some of the parameters discussed above.

What is claimed is:

]l. A process for forming ferrous metal powder compacts into high strength finished parts by heating and sintering each compact and thereafter hot forming the compact in a die which increases the density and establishes the final shape of the part, said process being characterized by heating and sintering treatments which include the steps of:

preheating each compact sufficiently to raise the temperature level of material in its outer surfaces to at least 300F but less than the sintering temperature for the material,

subjecting the compact to an induction field which rapidly raises the temperature of the material of the compact to a sintering level,

maintaining the temperature of the entire compact at said sintering level for a sufficient length of time to deoxidize the compact and to cause powder particles of the compact to coalesce, and

cooling the compact from said sintering level to a temperature at which the compact can be immediately formed into a finished part.

2. The process of claim I wherein said preheating step is carried out at a temperature level which raises the temperature of the material of the compact to at least 400 F.

3. The process of claim ll wherein said preheating step is carried out in a radiant heating zone at a temperature level in a range of about l,200 F to 2,000 F for a time period of about 30 seconds, or more depending upon the size of the compact.

4. The process of claim 1 wherein said step of subjecting said compact to an induction field raises the temperature of the compact to over 1,800" F.

5. The process of claim 4 wherein said induction field raises the temperature of said compact to about 2,350 F but to less than 2,500 F.

6. The process of claim 4 wherein said step of maintaining the temperature of the compact at said sintering level is carried out in a radiant heating zone.

7. The process of claim 11 wherein said cooling step reduces the temperature of the compact to a temperature of l,500l,900 F just prior to being formed.

8. The process of claim 1 wherein said cooling step reduces the temperature of at least portions of said compact to a level below the temperature required for forming, and including a subsequent step of reheating the entire compact to the temperature desired for forming.

9. The process of claim 1 wherein each compact is subjected to said heating and sintering treatments in a continuous process which subjects the compact to preheating of its surfaces to about 400 F, followed by rapid induction heating of the entire compact to about 2,300 to 2,400 F, followed by continued heating and sintering at about 2,300 F to 2,400 F, followed by an adjusting of the temperature of the entire compact to about 1,850 F, after which the compact is formed while hot and before there is a reoxidation of the material of which it is composed.

10. The process of claim 1 wherein said steps of heating and sintering are carried out in a reducing atmosphere.

lll. A process for increasing the rate at which ferrous metal powder compacts can be efficiently heated in an induction field, comprising the steps of preheating the powder metal compact sufficiently to raise the temperature level of the material in its outer surfaces to at least 300F but less than the sintering temperature for the material, and thereafter subjecting the compact to an induction field which rapidly raises the temperature level of the material of the compact to a level at which the compact can be sintered.

12. The process of claim 11 wherein said preheating step is carried out at a temperature level which raises the temperature of the material of the compact to at least 400 F.

13. The process of claim lll wherein said preheating step is carried out in a heating zone at a temperature level in a range of about 1,500 F to 2,000 F and for a time period of about 30 seconds, or more, depending upon the size of the compact.

14. The process of claim 11 wherein the temperature level of the material of said compact is raised to a temperature between 2,050 and 2,400 F in said induction field.

15. The process of claim l4 wherein induction heating is carried out in a field having a current frequency of at least 1 KH 16. The process of claim 14 and including a step of maintaining the temperature of said compact at a level higher than l,800 F for a sufficient period of time to sintcr the compact after said step of subjecting the compact to an induction field.

17. The process of claim 14 wherein the temperature level of the material of said compact is raised to about 2,350 F in said induction field and thereafter maintained at about 2,350 F for a sufficient period of time to sinter the compact.

18. The process of claim 17 and including a step of adjusting the temperature of said compact, after it has been sintered, to condition the compact for a lower temperature forming operation.

19. The process of claim ill wherein said compact is allowed to cool after said preheating step and before being subjected to induction heatingv 20. The process of claim 11 wherein said compact is subjected to induction heating immediately after said preheating step in a continuous process which subjects the compact to sintering temperatures before a final hot forming operation.

21. The process of claim 11 wherein said preheating step is carried out in a sufficiently short time span to raise the temperature of only the outer surface of said compact to at least 200 F. 

2. The process of claim 1 wherein said preheating step is carried out at a temperature level which raises the temperature of the material of the compact to at least 400* F.
 3. The process of claim 1 wherein said preheating step is carried out in a radiant heating zone at a temperature level in a range of about 1,200* F to 2,000* F for a time period of about 30 seconds, or more depending upon the size of the compact.
 4. The process of claim 1 wherein said step of subjecting said compact to an induction field raises the temperature of the compact to over 1,800* F.
 5. The process of claim 4 wherein said induction field raises the temperature of said compact to about 2,350* F but to less than 2,500* F.
 6. The process of claim 4 wherein said step of maintaining the temperature of the compact at said sintering level is carried out in a radiant heating zone.
 7. The process of claim 1 wherein said cooling step reduces the temperature of the compact to a temperature of 1,500*-1,900* F just prior to being formed.
 8. The process of claim 1 wherein said cooling step reduces the temperature of at least portions of said compact to a level below the temperature required for forming, and including a subsequent step of reheating the entire compact to the temperature desired for forming.
 9. The process of claim 1 wherein each compact is subjected to said heating and sintering treatments in a continuous process which subjects the compact to preheating of its surfaces to about 400* F, followed by rapid induction heating of the entire compact to about 2,300* to 2,400* F, followed by continued heating and sintering at about 2,300* F to 2,400* F, followed by an adjusting of the temperature of the entire compact to about 1,850* F, after which the compact is formed while hot and before there is a reoxidation of the material of which it is composed.
 10. The process of claim 1 wherein said steps of heating and sintering are carried out in a reducing atmosphere.
 11. A process for increasing the rate at which ferrous metal powder compacts can be efficiently heated in an induction field, comprising the steps of preheating the powder metal compact sufficiently to raise the temperature level of the material in its outer surfaces to at least 300*F but less than the sintering temperature for the material, and thereafter subjecting the compact to an induction field which rapidly raises the temperature level of the material of the compact to a level at which the compact can be sintered.
 12. The process of claim 11 wherein said preheating step is carried out at a temperature level which raises the temperature of the material of the compact to at least 400* F.
 13. The process of claim 11 wherein said preheating step is carried out in a heating zone at a temperature level in a range of about 1,500* F to 2,000* F and for a time period of about 30 seconds, or more, depending upon the size of the compact.
 14. The process of claim 11 wherein the temperature level of the material of said compact is raised to a temperature between 2, 050* and 2,400* F in said induction field.
 15. The process of claim 14 wherein induction heating is carried out in a field having a current frequency of at least 1 KHz.
 16. The process of claim 14 and including a step of maintaining the temperature of said compact at a level higher than 1,800* F for a sufficient period of time to sinter the compact after said step of subjecting the compact to an induction field.
 17. The process of claim 14 wherein the temperature level of the material of said compact is raised to about 2,350* F in said induction field and thereafter maintained at about 2,350* F for a sufficient period of time to sinter the compact.
 18. The process of claim 17 and including a step of adjusting the temperature of said compact, after it has been sintered, to condition the compact for a lower temperature forming operation.
 19. The process of claim 11 wherein said compact is allowed to cool after said preheating step and before being subjected to induction heating.
 20. The process of claim 11 wherein said compact is subjected to induction heating immediately after said preheating step in a continuous process which subjects the compact to sintering temperatures before a final hot forming operation.
 21. The process of claim 11 wherein said preheating step is carried out in a sufficiently short time span to raise the temperature of only the outer surface of said compact to at least 200* F. 